Novel communications and/or navigation systems by means of satellite are making ever-increasing use of satellites that are in non-geostationary orbits, e.g. satellites in low earth orbit (LEO), or satellites in medium or high earth orbit (MEO, HEO, . . . ), which orbits may be circular or elliptical, and implementing a large number of such satellites in order to ensure complete coverage of a geographical zone of interest which is generally worldwide or multi-regional. In such systems, the satellites are generally all on similar orbits, i.e. orbits having the same altitude and the same inclination.
Certain frequency bands, e.g. the Ku, Ka, L, etc. bands, are most advantageous for such novel systems, but they suffer from the drawback of also being used by numerous satellites for telecommunications and/or navigation systems that are in geostationary orbit.
It follows that radio links established from such non-geostationary satellites of such novel systems need to comply with the frequency-coordination rules laid down by the National Telecommunications Union so as to avoid interfering with or being interfered by other systems, whether in space or otherwise. FIG. 1 shows the geometrical conditions for radio interference on a link between a terrestrial user U and one or more satellites SG1 or SG2 in the geostationary belt C by means of a non-geostationary satellite SNG. Interference exists if there is sufficient alignment between the user, the non-geostationary satellite, and a geostationary satellite, as shown in FIG. 1 by the two lines B1 and B2.
Frequency coordination requires the operator of any such new system to ensure that the radio link established between a non-geostationary satellite and the surface of the terrestrial globe is interrupted whenever the satellite is in alignment with a geostationary satellite and a terrestrial user (to within a certain tolerance), and that contributes to increasing the number of non-geostationary satellites that are needed for providing continuous service over a geographical zone of interest, and therefore results in an increase in the cost of such novel systems.
Numerous studies have already been performed on the problem of complete and permanent coverage of the terrestrial globe by non-geostationary satellites. These studies have led to so-called "symmetrical" satellite constellations such as those of Messieurs Walker, Rider, Ballard, or others. In such known constellations, the orbital planes and the positions of non-geostationary satellites are uniformly distributed. In particular, satellites situated in the same orbital plane are thus equidistant and the orbital planes are uniformly distributed in the terrestrial equatorial plane (Walker and Ballard) or in the terrestrial equatorial half-plane (Rider).